Carbon membranes, their preparation and their use in the separation of various gases are known in the art, e.g., from U.S. Pat. No. 4,685,940, United Kingdom Patent No. 2,207,666 and European Patent No. 621,071. These membranes have been used for the separation of gas mixtures resulting from various processes. The most common process to which such methods have been applied are the separation of nitrogen and oxygen from air, but the separation of various binary gas mixtures including N.sub.2, He, O.sub.2 and CO.sub.2 have also been carried out. Recently U.S. Pat. No. 5,104,425 has taught the separation of hydrogen from hydrocarbons using a carbon membrane, but those membranes separate based on differences in adsorptive power and surface diffusively of the different molecules such that often larger molecules (ethane and propane) permeate better than smaller molecules (hydrogen). The effect is often weakened or lost at higher temperatures. While the prior art, and specifically the above-mentioned U.S. Pat. No. 4,686,940, broadly mentions a pore size of the carbon membranes in the range of 2.5 .ANG. to 5.0 .ANG., no membranes with pore sizes of above 3.8 .ANG. have actually been prepared, because the separation problems contemplated by the prior art did not include difficult separations, such as the separation of isomers, and did not address gases having size which exceeds 3.8 .ANG.. It is clear, therefore, that there is a need for a carbon membrane, characterized by a separation parameter which is solely dependent on the molecular dimension of the molecules to be separated; namely, the larger is the molecular dimension the lower is its permeability, and that said membrane should allow the separation of molecules, dimension of which is larger than 3.8 .ANG..
The art teaches different methods for separating branched from linear hydrocarbons, which do not involve carbon membranes. U.S. Pat. No. 5,069,794 and U.S. Pat. No . 2,924,630 disclose the use of molecular sieves zeolites, which are typically metallo alumino silicates. The pores in the crystalline structure of these molecular sieves have appropriate diameters to allow the separation. However, the preparation of said sieves is rather cumbersome, as it involves growing the porous crystalline structure over an appropriate substrate and then removing the layer of the crystalline zeolite, or providing the zeolitic barrier material the form of a filter cake, trapped between supporting surfaces. Furthermore, the performance of the carbon membrane according to the present invention is significantly better. For instance, the selectivity value for the separation of 2,2-dimethylbutane from n-hexane according to U.S. Pat. No. 5,069,794 is approximately 17. According to the present invention, a selectivity value, number of magnitudes of order higher, is obtained when separating 2,2-dimethylpropane from n-pentane.